I agree with Serum, 6S in a buggy would be just too much. How fast do you want to go. If speed is what you're after, maybe a buggy isn't the proper host for the conversion.

I've done two buggies in the last month or so and we're running a mamba max in each, one with an 8XL and the other with a 7XL. They are plenty fast for the track or bashing. The speed difference between the two are negligible with the 8XL geared 16/46 and the 7XL geared 14/46.

If you do choose to go with the 5S, then maybe a 9XL or maybe even an 8XL would be in order. An 8XL on 5S would be stupid fast for a buggy.